verslagen1 wrote on 06/04/10 at 11:21:36:How and when doe it backfire?
On shut down... turn the idle mixture screw about 1/8 turn CCW richer until it goes from a kerpow to a poof. Don't make it too rich.
On gear shift... tighten header bolts and look for air leaks in the intake and exhaust. WD40 for the intake and a lit candle for the exhaust. Don't reverse the two
Also, cleaning and/or adjusting the spring on the air shutoff valve may work too.
you know, i've lurked on this site for a long time, and i think that is the most simple, best explanation in short i've ever heard.
b-p-stewart, if those don't work, i suggest using the search function on the site for "backfire" or "afterfire" and set the time limit for a few months and you'll see dozens of posts about it, but they all pretty much boil down to what verslagen said.
check yer carb for proper tuning, check yer exhaust for leaks, ride on.
if you need advice about getting to the idle mixture screw, just search for that the same way. to sum up, there is a brass plug on the right side of the carb you (carefully) drill out and remove to get to a small brass screw that adjusts the idle mixture on the carb.